Variable pitch narrow band noise generator

ABSTRACT

A variable pitch narrow band noise generator comprising a noise signal source for generating a noise signal containing broad band frequency components; an amplifier for amplifying the noise signal from the source; a filter consisting of a plurality of frequency-selecting circuits supplied in common with the amplified noise signal from the amplifier, selecting different band-pass frequency components and connected in parallel in the order of the center frequency of frequency ranges of the bandpass frequency components; a normally open type variable resistor assembly consisting of an elongated resistor element and a similarly elongated flexible conductive contact element disposed adjacent to each other; a signal-circulating circuit for feeding a noise signal derived from the filter with depression of the flexible conductive contact element of the variable resistor assembly back to the amplifier so as to produce the noise signal in a narrower band form; and an output amplifier for amplifying the resultant narrow band pitch noise signal obtained from the signal-circulating circuit, whereby there can be easily obtained particularly from the output amplifier a noise signal having such a pitch sensation as is observed in a husky voice and containing a certain narrow band frequency components, the signals being capable of varying in pitch continuously as desired.

United States Patent [72] Inventor Takeslli Adachi Primary Examiner-Milton O. Hirshfield Hamamatsu, Japan Assistant ExaminerUlysses Weldon [2]] Appl. No. 65.240 Attorney-Flynn & Frishauf [22] Filed Aug. 19, 1970 [45] Patented Sept. 28, 1971 [7 3] Assignee Nippon Gakki Seizo Kabushiki Kaisha Y Hamama'sl'shihpan ABSTRACT: A variable itch narrow band noise enerator [32] Pnomy g i 2g 1969 comprising a noise signal source for generating a no ise signal gp 19 gontainilnfg broald band freqqefncy components; an amplifier or amp i ying t e noise signa mm the source; a filter consist- [311 44/ 66103 44/ 66104 and ing of a plurality of frequency-selecting circuits supplied in common with the amplified noise signal from the amplifier, selecting different band-pass frequency components and con- [54] VARIABLE PITCH NARROW BAND NOISE nected in parallel in the order of the center frequency of GENERATOR frequency ranges of the band-pass frequency components; a 1 1 Claims, 9 Drawing Figs normally open type variable resistor assembly consisting of an elongated resistor element and a similarly elongated flexible [52] US. Cl 84/L0l, conductive Contact element disposed adjacent to each h a 84/1-0584/1-1184/11934/124331/78 signal-circulating circuit for feeding a noise signal derived [51] Int. Cl Gl0h 1/02 from the filter with depression of the flexible conductive [50] Field of Search 84/1 .Ol tact element f the variable resistor assembly back to the 1'03, 1-24; 331/78; 340/384 392 plifier so as to produce the noise signal in a narrower band form; and an out ut am lifier for am Iif in the resultant nar- [56] References Cited row band pitch n lise signal obtained fro m fhe signal'circulat- UNITED STATES PATENTS ing circuit, whereby there can be easily obtained particularly 2,036,691 4/1936 Gourov 84/l.0l from the output amplifier a noise signal having such a pitch 2,245,598 6/ 1941 Llewellyn 84/1.05 X sensation as is observed in a husky voice and containing a cer- 2,593,442 4/1952 Hakkarinen 84/I.01 X tain narrow band frequency components, the signals being 3,254,336 5/1966 Campbell 84/1 .03 X capable of varying in pitch continuously as desired.

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SHEET 5 [IF 8 mom Nmm F W PATENTED 8EF28 I971 SHEET 6 OF 8 VARIABLE PITCH NARROW BAND NOISE GENERATOR BACKGROUND OF THE INVENTION The present invention relates to a variable pitch narrow band noise generator and more particularly a novel type of such generator which, when incorporated in an ordinary electronic musical instrument, enables musical sounds (or tones) to be played in greater variety.

Among electronic musical instruments, for example, an electronic organ or piano, there has been more favorably accepted in recent years such type as is capable of producing as great a variety of musical sounds as possible by a simplest possible operation in addition to those derived from original organs or pianos.

The instruments of the aforementioned kind which have heretofore put to practical use include the type which produces not only musical sounds such as melodious or accompaniment sounds of prescribed pitch obtained by selective operation of a plurality of keys arranged in the order of pitches, but also other sounds resembling those derived from desired particular natural musical instruments such as percussion, stringed and/or wind types either by varying the colors of tone signals using a special hannonic synthesizer or tonecoloring filter or by incorporating circuits for displaying special musical effects such as vibrato, tremolo, percussion, sustain, reverberation, attack, etc.

However, all the musical sounds of the prior art instruments are generally given forth in response to the keys depressed, and consist of fundamental tone signals having the prescribed pitches which only vary stepwise or discontinuously in accordance with the order in which the keys are arranged. Further, such fundamental tone signals are modified simply by mixing therewith other suitable frequency components in a proper ratio or by changing the amplitude or waveform of the fundamental tone signals. Accordingly, the conventional in strument has failed to produce such musical sounds as resemble voices or sounds which have a rather indefinite pitch as is observed in, for example, husky voices or breath sounds released from a wind instrument and contain a certain narrow band of frequency components. The prior art instrument has also presented difficulties in playing intermittently sounded musical tones simulating the chirpings or twitterings of birds.

SUMMARY OF THE INVENTION The present invention has been accomplished in view of the aforementioned situation and is intended to provide a variable pitch narrow band noise generator particularly adapted for use in a finger-board electronic musical instrument which can easily produce a narrow band pitch noise signal having a certain pitch sensation as is observed in husky voices or breath sounds released from a wind instrument or other signals simulating intermittently sounded tones such as the chirpings or twitterings of birds and can also continuously vary the pitch of said special signals.

Namely, the variable pitch narrow band noise signal of the present invention is obtained by amplifying a noise signal from a noise signal source for generating a broad band noise signal by an amplifier and supplying the amplified signal in common to a plurality of frequency-selecting circuits connected in parallel in the order of the center frequency of frequency ranges possessed by different band-pass frequency components so as to select them as desired. In this case, either of the input and output terminals of each of said frequencyselecting circuits is connected to the tap points on the resistor element involved in the resistor assembly consisting of an elongated resistor element and a similarly elongated flexible conductive contact element closely facing each other, said tap points being arranged substantially at an equal space in the longitudinal direction of the resistor element. The band-pass frequency components of the amplified noise signal obtained through the frequency-selecting circuits by depression of a given point on the longitudinal plane of the contact element of the variable resistor assembly are properly selected to form a desired pitched noise signal (i.e. a noise signal having a pitch sensation). There is further provided a signal-circulating circuit for feeding the pitched noise signal back to the amplifier making a feedback loop so as to render the signal more pitchsensational. Thus the resultant narrow band pitched noise signal is derived from an output amplifier. A noise generator of the aforementioned arrangement easily generates from said output amplifier signals having such a semidefinite pitch as is observed in a husky voice and containing certain narrow band frequency components. The pitch of the signal from the generator can be varied continuously as desired by shifting a pointof depression on the contact element along its longitudinal plane. In this case, it will be apparent that the greater the amplification gain in the signal-circulating circuit (i.e. the loop gain of the circuit), the more elevated the pitch sensation of the signals derived from the output amplifier and vice versa.

According to the present invention, there is disposed between the output amplifier and signabcirculating circuit a gain control circuit for negatively feeding the rectified voltage obtained by rectifying output voltage from said amplifier back to the signal circulating circuit, thereby more stabilizing the operation of the circuitry of the noise generator. Further in the gain control circuit is provided a time constant circuit consisting of a capacitor and a DC impedance element so as to charge and discharge current. The charging and discharging action of the time constant circuit enables intermittently sounded signals resembling bird voices to be easily obtained from the output amplifier.

FIG. I is a schematic circuitry of a variable pitch narrow band noise generator according to an embodiment of the present invention;

FIG. 2 is a concrete pictorial view of a variable resistor assembly involved in the circuitry of FIG. 1;

FIG. 3 shows a concrete circuit diagram of the variable pitch narrow band noise generator of FIG. I;

FIG. 4 is a schematic circuitry of a variable pitch narrow band noise generator modified from FIG. I;

, FIG. 5 is a schematic circuitry of a variable pitch narrow band noise generator according to another embodiment of the invention;

FIG. 6 represents a concrete circuit diagram of FIG. 5;

FIG. 7 is a schematic circuitry of a variable pitch narrow band noise generator according to still another embodiment of the invention;

FIG. 8 is a concrete circuit diagram of FIG. 7; and

FIG. 9 is a schematic circuitry of a variable pitch narrow band filter on which the present invention is based.

DESCRIPTION OF THE PREFERRED EMBODIMENTS There will now be described by reference to the appended drawings a variable pitch narrow band noise generator according to the preferred embodiments of the present invention.

FIG. I is a schematic circuitry of one of said embodiments. According to this embodiment, noise signals from a noise source 11 for generating broad band noise signals are supplied, after being amplified by an amplifier 12, to a filter 13 consisting of a plurality of, for example, four frequency-selecting circuits I31, 132, I33 and 134 selecting different bandpass frequency components and arranged in the order of the center frequency of frequency ranges possessed by the components. The output terminals of the frequency-selecting circuits 131 to I34 constituting the filter 13 are connected to the tap points on the later-described elongated resistor element l4 substantially equally spaced in its longitudinal direction. In a plane adjacent to that of the resistor element 14 is positioned a similarly elongated flexible conductive contact element 15. When there is depressed a given point on the longitudinal plane of the contact element 15 to contact the corresponding part of the resistor element 14, then there is obtained from the contact element I5 through the filter 13 a pitched noise signal containing the band-pass frequency components selected by the depression point. The resistor element 14 and flexible contact member constitute a nonnally open type resistor assembly 16 whose resistivity is determined by that point on the contact member 15 at which it is depressed and which can vary the band-pass frequency components of a noise signal obtained from the filter 13 according to the magnitude of resistivity thus determined. Accordingly, the shifting of the depression point along the longitudinal plane of the contact element 15 enables the band-pass frequency components of noise signals derived from the filter 13 to be changed in pitch continuously as desired.

Pitched noise signals generated by the variable resistor assembly 16 upon depression of the contact member 15 are amplified by an amplifier 17 and fed back to the amplifier 12 together with noise signals produced by the noise source 11, thereby fonning a signaLcirculating circuit 18 looped as the noise source 11: amplifier 12 ;filter 13 variable resistor assembly 16: amplifier 17: back to the amplifier 12. Output pitched noise signals from the signal-circulating circuit 18 are derived from an output amplifier 19.

In this case, it may be contemplated to use pitched noise signals generated by depression of the contact element 15 as output from the output amplifier 19 by allowing said signals to pass through the filter 13 only once, instead of circulating them through the signalcirculating circuit 18. When reproduced from a loudspeaker, however, such signals generally present noise characteristics too prominently (i.e. too broad bandwidth) to be available as musical sounds.

In contrast, employment of the signal-circulating circuit 18 (assuming now that the loop gain of the circuit is set at less than 1) permits noise signals generated upon depression of the contact member 15 to pass the filter 13 repeatedly, rendering output from the output amplifier 19 sufficiently pitch sensational to be practically used as musical sounds. It will be apparent that signals from the output amplifier 19 will have an increasing pitch sensation as the signal circulating circuit 18 causes the filtering band width to be narrower. These looped circuits except the source 11 constitute a very sharp narrow band filter when the loop gain is set at less than 1. And when the loop gain is not set at less than 1, the whole circuit will constitute a self-oscillating circuit, providing a signal of a definite pitch tone plus pitched noise.

FIG. 9 represents a schematic circuitry of the looped circuitry or a variable frequency narrow band filter except the source 11 used in the circuitry of FIG. 1. This circuitry involves an input terminal for supplying signals having a desired waveform, for example, a sine, rectangular or triangular wave and an output terminal 20 positioned on the output side of the output amplifier 19. It will be apparent that the circuitry of FIG. 9 will enable desired variable frequency narrow band signals corresponding to the waveform of signals supplied from the input terminal 10 to be derived from the output terminal 20 as the result of the aforementioned operation of the variable resistor assembly 16.

FIG. 2 is a concrete pictorial view of the aforementioned variable resistor assembly 16. To fabricate this resistor assembly 16, there is provided an insulating member 23 of a substantially rectangular shape in cross section having a relatively thin-walled portion 21 with a flat surface extending along the central line of the longitudinal plane on one surface thereof and another portion progressively thickening toward both edges starting from said flat plane 21 so as to form a pair of taper wall planes 22a and 22b. To the flat plane 21 of the insulating member 23 is mounted an elongated resistor member 141 having tap points. There is further mounted a flexible cover 24 in such a manner as to close up entirely that surface of the insulating member 23. On that part of the underside of the cover 24 which faces the resistor element 141 is fitted a flexible conductive contact element 151 of substantially the same shape and size as the resistor element with a very small space allowed between the resistor element 141 and contact element 151.

The variable resistor 161 is only closed when that part of the cover 24 which faces the resistor 141 is depressed by the finger in the direction of the arrow 25 to cause that part of the contact element 151 which faces the depression point to contact the corresponding part of the resistor element for conduction. At this time the variable resistor 161 presents a value of resistance determined by the depression point.

P10. 3 is a concrete circuit diagram of a variable pitch narrow band noise generator according to the present invention shown in FIG. 1. The noise source consists of a white noise source 33 and an amplifier 34. The white noise source 33 involves NPN-type transistor TR whose base is connected to the grounded terminal 31'; of a B power source through a resistor R and whose emitter is connected to the terminal 31p of a +8 power source. The amplifier 34 consists of two cascade-connected NPN-type emitter-grounded transistors TR and TR;, to amplify white noise signals from the source 33. Amplified white noise signals from the source 11a pass through a variable resistor assembly VR for adjustment of amplitude which is connected between the collector of transistor TR; and the source terminal 31p and then are supplied for further amplification to an amplifier 12a consisting of NPN-type emittevgrounded transistor TR The noise signals thus amplified are conducted to the base of NPN-type emitterfollower transistor TR Between the emitter of transistor TR, which is connected through a load resistor R to the grounded terminal 31!: and the source terminal 31p there are provided four parallel Wien bridge circuits 351, 352, 353 and 354 which are formed by serially connecting four serial circuits involving four capacitor-resistor groups of C ,-R,, C -R C -R, and C, --R, and four parallel circuits involving capacitor-resistor groups of C R C -R C -R and C R To the output terminals constituted by the junctures of the four serial circuits and four parallel circuits involved in the Wien bridge circuits 351 to 354 are connected the bases of NPN-type emitter-follower transistors TR TR,, TR and TR for impedance matching through serially connected coupling capacitors C C C and C The emitters of transistors TR, to TR, which are connected to the grounded terminal 311: through load resistors R R R and R respectively are further connected to the points on the aforesaid resistor 14 arranged substantially at an equal space in its longitudinal direction. Thus are formed four frequency-selecting circuits 131a, 132a, 133a and 134a by combining the Wien bridge circuits 351 to 354 and emitterfollower transistors TR to TR,. Signals generated upon depression of the contact member 15 are amplified by an amplifier 17a involving emitter-grounded transistor TR and then conducted through a semistationary resistor VR for adjustment of amplitude back to the aforementioned transistor TR thereby forming a signal-circulating circuit 180.

On the other hand, resultant narrow band pitched noise signals obtained from the signal circulating circuit are amplified by an output amplifier 19a consisting of three cascadeconnected transistors TR TR and TR and then are drawn out from an output terminal 36 through a coupling capacitor C With a variable pitch narrow band noise generator arranged as described above, the band-pass frequency components of noise signals derived from the variable resistor assembly 16 upon depression of the contact element 14 are determined in the following manner. When the point on the contact element 15 at which it is depressed for contact with the resistor element 14 corresponds to any of the contacts of the emitters of transistors TR, to TR, involved in the frequency-selecting circuits 131a to 134a, then the aforesaid band-pass frequency component is selected only by the corresponding one of the Wien bridge circuits 351 to 354. However, where the depression point on the contact element 15 for contact with the resistor element 14 is disposed intennediate between the contacts of the emitters of any two adjacent ones of transistors TR, to TR then the band-pass frequency component is determined by the corresponding two Wien bridge circuits in inverse ratio to the value of resistance prevailing across said two emitter contacts. Referring to FIG. 3, capacitors C C and C connected between the collectors and bases of transistors TR TR and TR act as neutrodynes so as to suppress the occurrence of undesirable oscillation in the high-frequency regions thereof. Resistors R R and R connected between the emitters of transistors TR,, TR. and TR and the bases of their preceding transistors TR,, TR, and TR are DC biasing resistors for transistors TR,, TR and TR FIG. 4 is a schematic circuitry of a variable pitch narrow band noise generator modified from FIG. 1. In FIG. 1, the variable resistor assembly 16 was disposed on the output side of the filter 13. It will be apparent, however, that the present invention may be practiced in the same way and with the same effect as in FIG. 1 even by placing said resistor assembly 16 on the input side of the filter 13 as shown in FIG. 4.

FIG. 5 is a schematic circuitry of a variable pitch narrow band noise generator according to another embodiment of the present invention. This embodiment additionally involves a gain control circuit 41 for restricting an amplification gain in the signal-circulating circuit 18 to less than 1 in accordance with the voltage level of output signals from the output amplifier 19. This stabilizes the loop gain of the circuit.

FIG. 6 is a concrete circuit diagram of FIG. 5. Namely, part of the output signals from transistor TR, is branched off to the base of NPN-type emitter-grounded transistor TR followed by amplification. The amplified signals from transistor TR are rectified by a rectifier 51 consisting of a diode D connected with an indicated polarity and then are supplied through serially connected resistor R to the gate terminal of a field-effect transistor FET (hereinafter referred to as FE whose drain and source terminals are connected between the emitter of transistor TR and the source terminal 31p.

With a variable pitch narrow band noise generator provided with the gain control circuit 41a arranged as described above, the higher the level of signals conducted through the signalcirculating circuit 180, namely, the voltage level of output signals from the output amplifier 19a, the more elevated the voltage level of rectified signals obtained from the rectifier 51. The resultant increased conductivity in the PET and in consequence the reduced resistance across its drain and source terminals act to decrease the amplification gain in the signal circulating circuit 18a. The aforementioned gain control circuit 41a serves as a sort of negative feedback circuit, namely, limits an amplification gain in the signal-circulating circuit 18a stably to less than 1, thereby surely preventing the occurrence of oscillation in said circuit 18a. Accordingly, the noise generator involving the gain control circuit of FIGS. 5 and 6 can be more stably operated than those of FIGS. 1 and 3.

FIG. 7 is a schematic circuitry of a variablepitch narrow band noise generator according to still another embodiment of the present invention. This embodiment includes a time constant circuit 61 for charging and discharging the rectified signals in connection with the gain control circuit 41. The charging and discharging action of said time constant circuit 61 enables signals representing intermittently sounded tones simulating bird chirpings to be produced from the output amplifier 19.

FIG. 8 is a concrete circuit diagram of FIG. 7. A time constant circuit 61a of FIG. 8 is formed by connecting a capacitor C, through a serially connected switch SW between the juncture 62 of the gate terminal of the FET with the resistor R and the grounded terminal 31n. Thus the time constant circuit 61a consisting of the capacitor C 1 and the resistor R acts to charge and discharge the rectified signals obtained through the diode d.

With a variable pitch narrow band noise generator of the aforementioned arrangement, depression of the contact element involved in the variable resistor assembly 16 with the switch SW closed allows output signals to be drawn out from the output amplifier 19a while the capacitor C is charged through the resistor R with signals rectified by the rectifier 51, whereas, when the charged voltage of the capacitor C exceeds a certain level, there occurs a short circuit across the drain and source terminals of the FET, preventing signals from being drawn out from the output amplifier 19a. Next when the charged voltage of the capacitor C is discharged through the resistor R the resistance between the drain and source terminals of the FET gradually increases until signals are again drawn out from the output amplifier 19a. The same operation as described above is repeated, intermittently obtaining signals from the output amplifier 19a.

What is claimed is:

1. A variable pitch narrow band noise generator comprising a noise signal source for generating broad band noise signal components; an amplifier for amplifying noise signals from the noise source; a filter including a plurality of frequency-selecting circuits supplied in common with amplified noise signals from the amplifier and selecting different band-pass frequency components; a variable resistor assembly including an elongated resistor element and a similarly elongated flexible conductive contact element which closely face each other so as to selectively determine the band-pass frequency components of noise signals derived from the filter upon depression of the contact element; a signal-circulating circuit for feeding signals obtained by operation of the variable resistor assembly back to the amplifier so as to form more pitch sensation signals; and an output amplifier for deriving resultant narrow band pitched noise signals obtained through the signal-circulating circuit.

2. The noise generator according to claim 1 wherein the noise signal source includes a white noise signal source.

3. The noise generator according to claim 1 wherein the frequency-selecting circuits constituting the filter are comprised of Wien bridge circuits.

4. The noise generator according to claim 1 wherein the variable resistor assembly comprises an elongated resistor element fitted substantially along the central line on the longitudinal upper surface of a substantially rectangular insulation member, a cover for closing up the upper surface of the insulation member in a manner closely to face the resistor element, the flexible conductive contact element being fixed to that part of the underside of the cover which faces the resistor element, whereby depression of that part of the upper surface of the cover which faces the contact element causes the part of the contact element at which it is depressed conductively to contact the corresponding part of the resistor element. thereby selectively determining the value of resistance of the resistor assembly in accordance with the position of the contact point.

5. The noise generator according to claim 1 wherein the signal-circulating circuit has its amplification gain set at less than 1, the output amplifier thereby providing narrow band pitched noise signals having a relatively low pitch sensation.

6. The noise generator according to claim 1 wherein the signal-circulating circuit has its amplification gain set at more than 1 to constitute a self-oscillation circuit, the output amplifier thereby providing narrow band pitched noise signals having a relatively high pitch sensation.

7. The noise generator according to claim 1 comprising a gain control circuit coupled between the output amplifier and signal-circulating circuit for negatively feeding a rectified voltage obtained by rectifying an output voltage from the output amplifier back to the signal-circulating circuit, thereby stabilizing the amplification gain therein to a value of less than one.

8. The noise generator according to claim 7 wherein the gain control circuit includes a rectifier for rectifying an output voltage from the output amplifier and a field-effect transistor whose gate electrode is supplied with the rectified voltage for variable control of the resistance across its source and drain electrodes, and in consequence an amplification gain in the signal-circulating circuit.

9. The noise generator according to claim 7 wherein the gain control circuit includes a time constant circuit which comprises a capacitor and DC impedance element so as to charge and discharge the rectified voltage, the charging and discharging action of the time constant circuit enabling signals to be drawn out intermittently from the output amplifier.

10. A variable frequency narrow band filter comprising an input terminal; an amplifier for amplifying signals from the input terminal; a filter including a plurality of frequencyselecting circuits supplied in common with amplified signals from the amplifier and selecting different band-pass frequency components; a variable resistor assembly including an elongated resistor element and a similarly elongated flexible conductive contact element which closely face each other so as to selectively determine the band-pass frequency components of signals derived from the filter upon depression of the contact 

1. A variable pitch narrow band noise generator comprising a noise signal source for generating broad band noise signal components; an amplifier for amplifying noise signals from the noise source; a filter including a plurality of frequencyselecting circuits supplied in common with amplified noise signals from the amplifier and selecting different band-pass frequency components; a variable resistor assembly including an elongated resistor element and a similarly elongated flexible conductive contact element which closely face each other so as to selectively determine the band-pass frequency components of noise signals derived from the filter upon depression of the contact element; a signal-circulating circuit for feeding signals obtained by operation of the variable resistor assembly back to the amplifier so as to form more pitch sensation signals; and an output amplifier for deriving resultant narrow band pitched noise signals obtained through the signal-circulating circuit.
 2. The noise generator according to claim 1 wherein the noise signal source includes a white noise signal source.
 3. The noise generator according to claim 1 wherein the frequency-selecting circuits constituting the filter are comprised of Wien bridge circuits.
 4. The noise generator according to claim 1 wherein the variable resistor assembly comprises an elongated resistor element fitted substantially along the central line on the longitudinal upper surface of a substantially rectangular insulation member, a cover for closing up the upper surface of the insulation member in a manner closely to face the resistor element, the flexible conductive contact element being fixed to that part of the underside of the cover which faces the resistor element, whereby depression of that part of the upper surface of the cover which faces the contact element causes the part of the contact element at which it is depressed conductively to contact the corresponding part of the resistor element, thereby selectively determining the value of resistance of the resistor assembly in accordance with the position of the contact point.
 5. The noise generator according to claim 1 wherein the signal-circulating circuit has its amplification gain set at less than 1, the output amplifier thereby providing narrow band pitched noise signals having a relatively low pitch sensation.
 6. The noise generator according to claim 1 wherein the signal-circulating circuit has its amplification gain set at more than 1 to constitute a self-oscillation circuit, the output amplifier thereby providing narrow band pitched noise signals having a relatively high pitch sensation.
 7. The noise generator according to claim 1 comprising a gain control circuit coupled between the output amplifier and signal-circulating circuit for negatively feeding a rectified voltage obtained by rectifying an output voltage from the output amplifier back to the signal-circulating circuit, thereby stabilizing the amplification gain therein to a value of less than one.
 8. The noise generator according to claim 7 wherein the gain control circuit includes a rectifier for rectifying an output voltage from the output amplifier and a field-effect transistor whose gate electrode is supplied with the rectified voltage for variable control of the resistance across its source and drain electrodes, and in consequence an amplificatioN gain in the signal-circulating circuit.
 9. The noise generator according to claim 7 wherein the gain control circuit includes a time constant circuit which comprises a capacitor and DC impedance element so as to charge and discharge the rectified voltage, the charging and discharging action of the time constant circuit enabling signals to be drawn out intermittently from the output amplifier.
 10. A variable frequency narrow band filter comprising an input terminal; an amplifier for amplifying signals from the input terminal; a filter including a plurality of frequency-selecting circuits supplied in common with amplified signals from the amplifier and selecting different band-pass frequency components; a variable resistor assembly including an elongated resistor element and a similarly elongated flexible conductive contact element which closely face each other so as to selectively determine the band-pass frequency components of signals derived from the filter upon depression of the contact element; a signal-circulating circuit for feeding signals obtained by operation of the variable resistor assembly back to the amplifier; an output amplifier for deriving filtered components of the signals from the signal-circulating circuit; and an output terminal for taking out the signals from said output amplifier.
 11. The noise generator according to claim 1 for use in an electronic musical instrument comprising keying means for operating the flexible conductive contact element in the variable resistor assembly for performance substantially in the same manner as a plurality of keys arranged in the order of pitches. 